1. Field of the Invention
This invention relates to a transient, rapid spectroscopic method for analysis of heterogeneous materials such as porous compounds of unknown composition, amalgamated powders or mineral samples.
2. Related Art
Most analytical techniques used in industry require taking samples to the laboratory, to be analyzed by time consuming procedures involving instrumentation such as Auger and mass spectrometers, EDS, liquid or gas chromatography, graphite furnace atomic absorption spectroscopy or inductively coupled plasma optical emission spectrometry. Faster in-situ methods such as spark-discharge optical spectrometry are only applicable to electrically conductive materials, while X-ray backscattering probes are limited in sensitivity.
An emerging method, laser induced plasma spectroscopy, promises to provide rapid, in-situ compositional analysis of a variety of materials in hostile environments and at a distance. Basically, this method includes focusing a high power pulsed laser on the material, thus vaporizing and ionizing a small volume of the material to produce a plasma having an elemental composition which is representative of the material composition. The optical emission of the plasma is analyzed with an optical spectrometer to obtain its atomic composition. This method has been applied to a variety of materials and industrial environments, as exemplified in the following documents.
U.S. Pat. No. 4,645,342 by Tanimoto et al. describes a probe for spectroscopic analysis of steel including focusing an infrared laser pulse on the steel material and collecting, at an angle of 16 degrees or more, the light emitted by the irradiated surface spot. The light is spectrally analyzed after a firstly emitted light (white noise) is excluded. The spectral background of the individual emissions is subtracted to obtain the net intensity of preset spectral lines representative of given elements, and the intensity of said spectral lines is related to the concentration of said elements in the steel material. The analyzed material, massive steel, has a good homogeneity. This cannot be said of heterogeneous materials such as porous materials where particle-to-particle interfaces represent randomly distributed thermal barriers which may considerably affect the plasma temperature and the amount of vaporized mass.
U.S. Pat. No. 4,986,658 to Kim describes a probe for molten metal analysis by laser induced plasma spectroscopy. The probe contains a high-power laser producing a pulse having a triangular pulse waveshape. When the probe head is immersed in the molten metal, the pulsed laser beam vaporizes a portion of the molten metal to produce a plasma having an elemental composition representative of the molten metal composition. Within the probe, there is provided a pair of spectrographs each having a diffraction grating coupled to a gated intensified photodiode array. The spectroscopic atomic emission of the plasma is analyzed and detected for two separate time windows during the life of the plasma using two spectrometers in parallel. The first time window analyzes the plasma plume before it reaches thermal equilibrium shortly after the termination of the laser pulse, typically 10 ns long, to detect line reversals, as caused by absorption of radiation emitted by the hotter inner portion of the plasma plume by relatively cooler outer portions of the plasma plume. Thereafter, after the plasma has reached thermal equilibrium, typically after 1 .mu.s, a second time window analyzes the more conventional line emissions from the optically emissive plasma. The spectra obtained during either the first or the second time window, or a combination of both, can be used to infer the atomic composition of the molten metal. Because the metal is molten, it has a good homogeneity. Again, this cannot be said for heterogeneous materials such as porous materials where particle-to-particle interfaces represent randomly distributed thermal barriers which may considerably affect the plasma temperature and the amount of vaporized mass.
U.S. Pat. No. 4,995,723 by Carlhoff et al. describes a similar probe for liquid metal analysis in a melting vessel. In this case, the pulsed laser beam and the fiber optic collector for the spectrometer are pointed to the liquid metal through a lateral opening in the vessel, through which opening a hot, inert gas is continuously injected to provide a clean and oxide-free molten metal surface to be analyzed by laser induced plasma spectroscopy. Again, the material is here homogeneous.
U.S. Pat. No. 5,042,947 by Potzschke et al. describes an application of laser induced plasma spectroscopy for the sorting of solid metal particles, namely shredder scrap from automotive recycling processes. Multiple laser pulses are here used to clean the surface from impurities, and up to 30 particles per second can thus be sorted depending on the resulting composition, typically aluminium, zinc, copper, lead and steel. Because here the purpose is sorting rather than precise compositional analysis, a relatively low precision and sensitivity can be accepted in this case.
Zigler in U.S. Pat. No. 5,379,103 describes a mobile laboratory for in-situ detection of organic and heavy metal pollutants in ground water. Pulsed laser energy is delivered via fiber optic media to create a laser spark on a remotely located analysis sample. The system operates in two modes, one being based on laser induced plasma spectroscopy and the other on laser induced fluorescence. In the first operational mode, the laser beam guided by optical fiber is focused by a lens on the sample to generate a plasma. The emitted spectrum is analyzed and used to detect heavy metals. In the second mode the focusing laser energy is removed allowing the laser beam via fiber optic to irradiate the sample, so that organic molecules with an aromatic structure emit absorbed ultraviolet energy as fluorescence. The emitted fluorescence light is transmitted via fiber optic media for further analysis. The measured wavelength and time characteristics of the emitted fluorescence can be compared against predetermined characteristics to identify the organic substances in the analysis sample. Zigler et al analyze trace quantities of both molecules and atoms in ground water. In the case of molecules, molecular spectra are analyzed using fluorescence. Again, the probed material, ground water, is homogeneous and compact, while the analysis of heterogeneous or porous materials must face signal variabilities requiring special techniques for improving their reliability, as described in the present disclosure.
The variability of the line emission signal detected in laser induced plasma spectroscopy originates from the fact that the intensity of the emission line of a given element i present in the plasma can be written as (see the paper by Chaleard et al., "Correction of matrix effects in quantitative elemental analysis with laser ablation optical emission spectrometry", Journal of Analytical Atomic Spectrometry, vol. 12, pp. 183-188, Febuary 1997): EQU I(i)=KC(i)Me.sup.-E/k
where K is a constant which takes into account the collection efficiency of the apparatus, C(i) is the concentration of element i in the plasma, which is the unknown to be measured, M is the mass of matter vaporized in the plasma plume, E is the atomic energy level to which the atom has to be raised in the plasma in order to provide, by de-excitation, the emitted intensity, k is Boltzmann's constant, and T is the average temperature of the plasma. E and k are known constants, while K can be established by calibration, so that in order to evaluate C(i) from a measurement of the emitted intensity I(i), the values of T and M need to be determined.
The analysis of heterogeneous and porous materials poses certain problems due to the random variability of the surface orientation in the small area (typically less than 1 mm in diameter), where the laser pulse happens to strike the surface, as well as to the possible presence of subsurface thermal barriers due to particle-to-particle interfaces which cool down the plasma by an amount unpredictable from shot to shot due to the correspondingly erratic local thermal conductivity of the material under the irradiated surface. Because of the above factors, neither T nor M are repetitive enough to be evaluated once and for all in the calibration stage, but may vary unpredictably from shot to shot or from one material to another. Methods for evaluating these two variables at each laser shot are thus required if this technique is to provide the required reliability.
The above mentioned paper by Chaleard et al provides an answer to these requirements. The temperature T of the plasma can be evaluated by the well known technique of establishing the ratio of two emission lines having different excitation energy levels and applying Boltzmann's law (see the above-mentioned paper at p. 186). As to the evaluation of the mass M of the matter vaporized in the plasma plume, it is estimated by using a microphone to detect the acoustic pulse produced by the expanding plasma induced by the laser shot. For flat metallic samples and under controlled laboratory conditions, it was demonstrated that the acoustic signal varied linearly, albeit with a small slope, with the amount of the mass ablated by the laser shot, such that by establishing a ratio of the detected intensity to the intensity of the acoustic signal, the normalized signal thus obtained was relatively independent of variations of the laser irradiance.
However, the utilization of an acoustic sensor for signal normalization has a number of drawbacks, particularly in the analysis of non-flat or porous materials:
a separate microphone is required to be positioned close to the irradiated sample, complicating the probe setup. The acoustic signal (differently from the optical emission signal which is collected by imaging the spectrometer slit, typically 0.1 mm wide, across the plasma which is viewed as a disk of typically 1 mm in diameter) varies as d.sup.-2, where d is the distance from the acoustic sensor to the irradiated spot. Consequently, the acoustic signal varies with the surface position, making it difficult to maintain signal repetitivity when scanning convoluted surfaces. Furthermore, the acoustic signal is typically affected by spurious reflections of the acoustic wave on an irregular sample surface or on nearby objects, while its intensity may be affected by possible variations of the air absorptivity due to variations in temperature, humidity etc; PA1 the acoustic pulse, due to the relatively large wavelength of the sound detectable by such microphones, integrates spatially and temporally across the full plasma plume and for the full duration of the plasma discharge. Consequently, the acoustic signal may be affected by spatial fluctuations of the random plasma plume outside the typically 0.1 mm wide slice of the plume which is imaged by the spectrometer. For this reason, in the above mentioned work (see the paper by Chaleard et al. at p. 184) a spatial filter was introduced in the laser beam to insure a flattened, "top hat" distribution. This severely reduces the useful laser power, while not assuring signal repetitivity in case of laser-to-spectrometer misalignment. Similarly, the acoustic signal integrates over the full, typically a few tens of .mu.s, duration of the plasma, while the detectors are gated to optimally respond only to a portion of this duration, within which portion the emission intensity may vary independently of the overall integrated plasma intensity.
The object of the present invention is to provide a method and apparatus for a reliable analysis of heterogeneous materials by laser induced plasma spectroscopy using a signal normalization which requires no physically separate sensor, so that both the signal and the normalizing factor are affected in an identical manner when scanning convoluted or irregular surfaces, or in the presence of fumes or other limitations to air transparency, while no unwanted spatial or temporal integration effects will take place.